It's not a simple answer, with multiple studies looking at different consumer behaviors and coming up with widely varying figures. But eMarketer analyzed a broad swath of them to come out with a pretty definitive estimate: Some 15% to 17% of viewers, or one-sixth of the audience, are engaging with each other on the web around TV content.

Whether that's a lot or a few is in the eye of the beholder. Either way, it's a behavior that's going to get a lot more prevalent. Just about every survey agrees that the younger the viewer, the more likely he or she is to interact with or around TV programming on a second screen. Among those under 35, more than half do so, according to the study:

Twitter has been making the case that its service is the only real forum for second-screen engagement, conversations on unfolding neither in public nor in real time. (Count among the persuaded: It's giving its subscribers the ability to tune into shows directly from tweets with the click of a single "See It" button.)

But a lot more people use Facebook while watching TV than use Twitter, even if they don't use it in quite the same way. In one survey of affluent consumers conducted by Ipsos Mendelsohn, 15% said they regularly use Facebook while watching, versus 6% for Twitter.

But thinking about this as a two-horse race may be a mistake -- and the third horse is more like an elephant. It turns out the most common second-screen activity isn't a social one per se. It's search. In a survey conducted in April, 41% of respondents said they have used the internet to look up information about a show they were watching. That's three times as many as wrote or read comments on a social network.